1. Field of the Invention
This invention relates to cement compositions suitable for the cementing of high temperature wells and methods of producing such compositions.
2. Description of the Prior Art
Before a well is finally completed and allowed to produce, the well casing is secured in place by cementing. The normal procedure is to pump an aqueous slurry of a hydraulic cement down through the well casing and then upwardly in the annular space between the casing and the walls of the wellbore. Upon curing, the cement serves two important functions: (1) it seals the casing annulus, preventing pressure communication between vertically spaced formations and (2) it anchors the casing in place in the well. Other types of well completion procedures such as remedial cementing also involve cementing operations.
Cements used in oilfield cementing are hydraulic cements, usually the Portland or Pozzolan classes or mixtures of these. The cements are produced by burning an intimate mixture of finely divided calcareous and argillaceous material and grinding the resultant residue to produce fine powder. Oilfield cements are classified by the American Petroleum Institute and should be used in accordance with the recommendations of that organization (see API specification for oilwell cement and cement additives, API standard 10-A, 16th Edition, April 1971).
The amount of water added to the cement to form the aqueous slurry varies between about 4 to 5 gallons per sack depending upon the API class. This concentration provides a slurry density between about 15.6 and 16.5 pounds per gallon. The rheological properties of the slurry are controlled by adding Bentonite or other additives to the slurry to increase its viscosity or adding thinners to reduce its viscosity.
With any well cementing procedure, the slurry must remain fluid for a sufficient length of time to allow completion of the job. A problem frequently encountered in cementing high temperature formations is early setting of the cement since temperatures accelerate the hydration of the cement components and thereby decrease thickening time. This problem is particularly acute with deep wells where formation temperatures frequently exceed 250.degree. F. In addition, deep wells require increased pumping time to pump the cement to greater depths. This combination of high temperature and increased pumping time necessitates the use of additives which retard the rate of thickening of the cement so that all of the cement can be pumped into place in the well before it begins to set.
Two commonly used cement retarder additives are calcium lignosulfonate and carboxymethyl hydroxyethyl cellulose (CMHEC). Calcium lignosulfonate is an effective retarder at moderate temperatures and also acts as a dispersant for other cement additives such as bentonite. CMHEC is used as a retarder and is also extensively employed for the purpose of reducing water loss to the formation. Normally, a small amount of the retarder is dry blended with the cement and an aqueous slurry of this mixture is then introduced into the well. The cementing procedure is then completed in the usual fashion.
The problem with these retardation compositions is that they are frequently unpredictable, especially at temperatures over 250.degree. F. It has been found that small changes in retarder concentration or formation temperature can result in unacceptably large variations in slurry thickening time and compressive strength development. On-site blending of retarder and cement adds to the problem because of the difficulty in thoroughly blending a small amount of retarder with a relatively large amount of cement. If not homogeneously blended, the retarder will be present in different proportions throughout the mixture resulting in unpredictable cement retardation and nonuniform compressive strength development.